The present invention relates to the removal of nitrogen oxides or xe2x80x9cNOxxe2x80x9d from exhaust gases and the like, and more particularly to processes and apparatus for reducing NOx selectively using autocatalytic, autothermal reactions in a manner to also remove other exhaust contaminants from the combustion of carbonaceous fuels, and also to industrial processes using the same.
Without being bound by any particular theory, the background of the present invention will be described by way of a description of particular problems discussed in the art and various proposed solutions to such problems. For brevity, various references will be briefly and generally summarized herein. A more complete understanding of such background art may be obtained by a complete review of the documents cited herein, etc. What should be understood from the following discussion is that, despite such extensive prior efforts to provide various methods of NOx removal and the like, a continuing need exists for practical and low-cost methods of NOx removal in a variety of industrial processes, which may utilize a variety of commercially-available reductants.
Carbonaceous fuels are burned in internal combustion engines and other equipment such as boilers, furnaces, heaters and incinerators, and the like (i.e., in a wide variety of industrial process). Excess air frequently is used to complete the oxidation of combustion byproducts such as carbon monoxide (CO), hydrocarbons and soot. High temperature combustion using excess air, however, tends to generate oxides of nitrogen (often referred to as NOx).
Emissions of NOx include nitric oxide (NO) and nitrogen dioxide (NO2). Free radicals of nitrogen (N2) and oxygen (O2) combine chemically primarily to form NO at high combustion temperatures. This thermal NOx tends to form even when nitrogen is removed from the fuel. Combustion modifications which decrease the formation of thermal NOx generally are limited by the generation of objectionable byproducts.
Mobile and stationary combustion equipment are concentrated sources of NOx emissions. When discharged to the air, emissions of NO oxidize to form NO2, which tends to accumulate excessively in many urban atmospheres. In sunlight, the NO2 reacts with volatile organic compounds to form groundlevel ozone, eye irritants and photochemical smog. These adverse effects have prompted extensive efforts for controlling NOx emissions to low levels. Despite advancements in fuel and combustion technology, groundlevel ozone concentrations still exceed federal guidelines in many urban regions. Under the Clean Air Act and its amendments, these ozone nonattainment areas must implement stringent NOx emissions regulations. Such regulations will require low NOx emissions levels that are attained only by exhaust aftertreatment.
Exhaust aftertreatment techniques tend to reduce NOx using various chemical or catalytic methods. Such methods are known in the art and involve nonselective catalytic reduction (NSCR), selective catalytic reduction (SCR) or selective noncatalytic reduction (SNCR). Alternatively, NO may be oxidized to NO2 for removal by wet scrubbers. Such aftertreatment methods typically require some type of reactant for removal of NOx emissions.
Wet scrubbing of NO2 produces waste solutions that represent potential sources of water pollution. Wet scrubbers primarily are used for NOx emissions from nitric acid plants or for concurrent removal of NO2 with sulfur dioxide (SO2). High costs and complexity generally limit scrubber technology to such special applications. Wet scrubbers are applied to combustion exhaust by converting NO to NO2, such as is described in U.S. Pat. No. 5,047,219.
The NSCR method typically uses unburned hydrocarbons and CO to reduce NOx emissions in the absence of O2. Fuel/air ratios must be controlled carefully to ensure low excess O2. Both reduction and oxidation catalysts are needed to remove emissions of CO and hydrocarbons while also reducing NOx. The cost of removing excess O2 precludes practical applications of NSCR methods to many O2-containing exhaust gases.
Combustion exhaust containing excess O2 generally requires chemical reductant(s) for NOx removal. Commercial SCR systems primarily use ammonia (NH3) as the reductant. Chemical reactions on a solid catalyst surface convert NOx to N2. These solid catalysts are selective for NOx removal and do not reduce emissions of CO and unburned hydrocarbons. Excess NH3 needed to achieve low NOx levels tends to result in NH3 breakthrough as a byproduct emission.
Large catalyst volumes are normally needed to maintain low levels of NOx and NH3 breakthrough. The catalyst activity depends on temperature and declines with use. Normal variations in catalyst activity are accommodated only by enlarging the volume of catalyst or limiting the range of combustion operation. Catalysts may require replacement prematurely due to sintering or poisoning when exposed to high levels of temperature or exhaust contaminants. Even under normal operating conditions, the SCR method requires a uniform distribution of NH3 relative to NOx in the exhaust gas. NOx emissions, however, are frequently distributed nonuniformly, so low levels of both NOx and NH3 breakthrough may be achieved only by controlling the distribution of injected NH3 or mixing the exhaust to a uniform NOx level.
NH3 breakthrough is alternatively limited by decomposing excess NH3 on the surface of a catalyst as described in U.S. Pat. No. 4,302,431. In this case, the excess NH3 is decomposed catalytically following an initially equivalent decomposition of NOx and NH3 together. The decomposition of excess NH3, however, reduces the selectivity of the SCR method, increasing the molar ratio of NH3 with respect to NOx as much as 1.5 or higher.
In a combination of catalytic and noncatalytic reduction methods, both NOx and NH3 removal may be controlled by SCR following an initial stage of NOx reduction by SNCR. In the SNCR method, NOx emissions may be reduced partially without controlling NH3 breakthrough to a low level. The SCR method may decrease NOx further while also lowering NH3 breakthrough to an acceptable level.
The use of excess NH3 to enhance NOx removal by the SNCR method is described in detail in U.S. Pat. Nos. 4,978,514 and 5,139,754. With such methods, the NH3 injection to SNCR is controlled so that the unreacted NH3 remains sufficient for the subsequent catalytic reduction of NOx to a low level. This injection strategy is based on the use of excess NH3 for reducing NOx to lower levels, as with the SCR method described above.
Another method for combining SNCR and SCR methods is described in U.S. Pat. No. 5,510,092. In this method, the catalytic NOx reduction is always maximized using a separate NH3 injection grid, and the NOx emissions are reduced noncatalytically only as needed to maintain a final low NOx level. This method decreases the consumption of NH3 by minimizing the use of SNCR which removes NOx less selectively than the catalytic method.
The low selectivity of the SNCR method and the use of excess NH3 for decreasing NOx levels is reported by Lyon, who is believed to have first suggested the noncatalytic reduction of NOx (U.S. Pat. No. 3,900,554). In commercial coal-fired boiler tests, 73% NO reduction has been reported with 2.2 ppm NH3 breakthrough using a 0.9 molar ratio of NH3 with respect to NO, while 86% NO reduction required 11 ppm NH3 breakthrough and a 2.2 molar ratio of NH3 with respect to NOx. These results are reported in Environ. Sci. Technol., Vol. 21, No. 3, 1987.
In another article (Ind. Eng. Chem. Fundam., Vol. 25, No. 1, 1986), Lyon also reports the inhibiting effect of NH3 on CO oxidation. This observation in experiments and commercial tests is confirmed by modeling studies. The inhibition has been explained in terms of competition between NH3 and CO for reaction with the OH free radical. It is believed that, while NH3 inhibits the oxidation of CO, the CO also decreases the selectivity of NO reduction by NH3.
This xe2x80x9csacrifice of residual CO oxidationxe2x80x9d is described by Lyon as an important limitation of the noncatalytic reduction method. According to these teachings, the injection of NH3 should follow the completion of CO oxidation in order to overcome this limitation. When NH3 is injected before the completion of CO oxidation, the oxidation of residual CO tends to diminish, which may result in greater levels of byproduct CO emissions.
Despite this disadvantage of greater byproduct CO emissions, many patents teach the use of CO or other ancillary reducing materials to lower the effective temperature for reducing NO noncatalytically. For example, the use of CO to lower the temperature for NO reduction by HNCO is discussed in U.S. Pat. No. 4,886,650 as follows: xe2x80x9cWhere it is desired to lower the operating temperature to a greater degree, larger amounts of CO or other H atom generating species will be added or vice-versa.xe2x80x9d
This lowering of the effective temperature for NO reduction has been repeated generally in a consistent manner throughout patent literature related to the noncatalytic method. The original discoveries of NH3 (U.S. Pat. No. 3,900,554) and urea (U.S. Pat. No. 4,208,386) as NOx reductants in the temperature range of 1600 to 2000xc2x0 F. both reported the use of ancillary reducing materials to enable noncatalytic NO reduction throughout the temperature range of 1300 to 2000xc2x0 F. Hydrogen (H2), CO, and hydrocarbon(s), including oxygenated hydrocarbons, have been proposed as ancillary reducing materials that may lower the effective temperature for noncatalytic NO reduction by NH3 or urea. This use of hydrocarbon(s) and CO is reportedly limited, however, due to incomplete oxidation, resulting in the production of air pollutants. Hydrogen has been cited as the preferred reducing material because it does not produce any air pollutants.
The use of hydrogen is limited because it decreases the selectivity for NO reduction by NH3 or urea. To overcome this limitation, the hydrogen may be added in successive multiple stages as described in U.S. Pat. No. 3,900,554. A more detailed description of a multi-stage method for noncatalytic NO reduction using NH3 and hydrogen is disclosed in U.S. Pat. No. 4,115,515. This multi-stage method typically requires two or more locations along the flowpath of the exhaust gas to inject reducing gas mixtures. The optimum use of multiple injection stages and alternative reducing gas mixtures depends on the exhaust gas temperature in the vicinity of each injection location. The multi-stage method accounts for temperature gradients along the gas flowpath as well as variations in temperature at each injection location.
Generally according to the these teachings, NH3 is injected as the only reducing gas at temperatures in the range of 900 to 1000xc2x0 C. (about 1650 to 1850xc2x0 F.), while mixtures of NH3 and hydrogen are injected at temperatures in the range of 700 to 900xc2x0 C. (about 1300 to 1650xc2x0 F). Decreasing the NO concentration in the first injection stage using NH3 alone minimizes the less-selective reduction of NO in the second stage where hydrogen is used as the ancillary reducing material.
Patents have proliferated since such disclosures of noncatalytic reduction methods. In particular, U.S. Pat. Nos. 4,731,231, 4,800,068, 4,886,650 and 4,908,193 have disclosed the decomposition of cyanuric acid, (HNCO)3, to generate isocyanic acid (HNCO) for NO reduction. Also, other patents (for example, U.S. Pat. Nos. 4,719,092, 4,751,065, 4,770,863, 4,803,059, 4,844,878, 4,863,705, 4,873,066, 4,877,591, 4,888,165,4,927,612 and 4,997,631) have disclosed a variety of reducing materials as alternatives to NH3, urea or cyanuric acid, or as enhancers for use with NH3, urea or cyanuric acid.
Such patents primarily address the acute problems of NH3 breakthrough and byproduct CO emissions that are characteristic of the noncatalytic reduction method. In addition to the disclosures of various reductants and enhancers, other patents (for example, U.S. Pat. Nos. 4,777,024, 4,780,289, 4,863,704, 4,877,590, 4,902,488, 4,985,218, 5,017,347 and 5,057,293) describe elaborate control strategies and multi-stage injection methods.
Such control strategies and multi-stage methods primarily address variations in temperature. Combustion equipment typically operate throughout a load range, and exhaust gas temperatures generally increase at higher loads. Therefore, local temperatures vary at the fixed locations where the reductant(s) and reducing material(s) are injected into the exhaust gas. The noncatalytic methods do not control the local temperature for NOx reduction.
With noncatalytic reduction methods, the local temperature typically is used as a means for controlling the injection of reductant(s) and ancillary reducing material(s). The patents teach the use of ancillary reducing material(s) to lower the effective temperature for NOx reduction so that it matches the actual local temperature, which depends solely on the production of the exhaust gas. It is important to note that, in such teachings, the ancillary reducing material(s) are not injected to control the local temperature. Ancillary reducing material(s) may enable NOx reduction at lower effective temperatures, but may result in the formation of objectionable byproducts. Such teachings tend only to minimize disadvantages of noncatalytic reduction methods. Such techniques in general do not provide for concurrent depletion of NH3 and CO emissions.
The use of oxygenated hydrocarbons is described in U.S. Pat. No. 4,830,839 as a means for scrubbing NH3 breakthrough from a previous stage of noncatalytic NOx reduction. With this method, vaporized concentrations of oxygenated hydrocarbons in the range of 2 to 500 ppm are added to the exhaust gas so that their weight ratios with respect to NH3 are in the range of 2 to 200. U.S. Pat. No. 5,047,219, however, subsequently discloses that oxygenated hydrocarbons oxidize NO to NO2 at temperatures below about 1600xc2x0 F.
Lowering the effective temperature for noncatalytic reduction below about 1700xc2x0 F. also slows the thermal decomposition of nitrous oxide (N2O) as described in U.S. Pat. No. 5,048,432. This patent teaches reheating of exhaust gases using a burner with a separate source of combustion air. The N2O is decomposed thermally when the high temperature flue gases from the burner mix and reheat the primary exhaust gas above 1700xc2x0 F.
Formation of N2O as a noncatalytic reduction byproduct is described in U.S. Pat. No. 4,997,631. When NOx emissions are reduced by the noncatalytic method, a portion of the reduced NOx is converted to N2O. As described above, N2O levels decrease at higher temperatures, but also the reported data suggest much less N2O formation when NH3 is used as the chemical reductant. Urea and cyanuric acid reportedly result in higher N2O levels.
A different method for staging noncatalytic NO reduction is described in U.S. Pat. No. 3,867,507. Hydrocarbons are disclosed to reduce NO when the molar ratio of O2 with respect to carbon is less than 2.5. Such low O2 levels, however, tend to result in the formation of objectionable byproducts, including NH3, hydrogen cyanide (HCN), CO and unburned hydrocarbons. Such byproducts are removed by oxidation using added air at elevated temperatures, e.g.,1100xc2x0 C. (about 2000xc2x0 F.), in a second stage.
Similar methods for staging noncatalytic NOx reduction are disclosed in U.S. Pat. Nos. 4,851,201 and 4,861,567. With these methods, reductant(s) are mixed with the exhaust gas and decomposed under fuel-rich combustion conditions in a first stage, and then NOx is reduced in a second stage with an excess of oxygen. The temperature and oxygen concentration are adjusted between the two stages. The temperature ranges for each stage depend on whether the reductant is cyanuric acid rather than NH3 or urea.
Another method for lowering the effective temperature for noncatalytic NOx reduction is described in Int. App. No. PCT/US92/07212 (Pub. No. WO 93/03998). It is suggested that hydrocarbons be injected so as to create stratified mixtures effective for generating partial oxidation products as ancillary reducing materials to lower the effective temperature for NOx reduction using cyanuric acid.
Based on such previous teachings, noncatalytic NOx reduction by hydrocarbon(s) alone tends to be limited to fuel-rich combustion conditions, i.e., low O2 levels. In the presence of excess O2, hydrocarbon(s) and CO are ineffective for NOx reduction, but may be used to lower the effective temperature for noncatalytic NOx reduction using chemical reductant(s). In such an ancillary role, such materials also are claimed to lower NH3 breakthrough, but such would be achieved only at the expense of decreased selectivity for NOx reduction.
Under these conditions, noncatalytic NOx reduction tends to be limited because excess NH3 breakthrough is needed to achieve low NOx levels, but the oxidation of CO is inhibited by NH3, so the lowering of noncatalytic reduction temperature using ancillary materials tends to increase byproduct CO emissions. Both NH3 and CO are objectionable SNCR byproducts. In general, even the most elaborate proposed SNCR controls and staging methods cannot deplete these objectionable byproducts concurrently. In general, such methods only minimize the disadvantageous production of one byproduct at the expense of increasing another.
Furthermore, noncatalytic reduction methods are highly dependent upon temperature, but tend to provide no means for controlling this key condition. Staging methods and elaborate controls are needed to maintain an effective temperature for the chemical reductant(s) when excess O2 is present at the local exhaust gas conditions. Injecting reductant(s) under specific fuel-rich combustion conditions also is claimed to require a staged introduction of excess air to complete the combustion of primary fuel.
Despite such staging methods and elaborate controls, the NH3 breakthrough from SNCR in general is depleted only by using a subsequent catalytic method. Such SCR methods, however, do not remove the objectionable byproduct CO emissions from SNCR without using a separate oxidation catalyst similar to NSCR. Alternatively, this disadvantage of SNCR may be minimized to the greatest extent by limiting noncatalytic reduction only to maintain a final low NOx level in combination with SCR as described in U.S. Pat. No. 5,510,092.
Relegating the inherent advantages of a gas-phase method for reducing NOx to a subordinate role, however, does not minimize the key disadvantages of using solid catalysts. The expensive installation of large catalyst volumes intrudes adversely upon the combustion equipment. The catalyst bed adds pressure drop, and the vaporization of NH3 may derate the combustion equipment by as much as 2%.
As is well known, solid catalysts tend to gradually become plugged and poisoned under normal use and require periodic replacement. Premature replacement is needed when the catalyst is sintered or poisoned due to unusually high temperatures or contaminant levels resulting from combustion-related problems or other equipment failures. Like SNCR methods, catalytic reduction methods are highly dependent upon on temperature, but provide no means for controlling this key condition.
In considering the foregoing teachings and efforts, and in particular in view of the continuing need in numerous applications for practical, cost-effective reduction of NOx despite the extensive previous efforts, a need exists for new methods for selective NOx reduction, which may in general combine certain advantages of previously disclosed methods substantially, but without disadvantages thereof.
As will be described hereinafter, Applicants submit that they have discovered such methods.
It should be understood that the foregoing discussion of the background of the present invention, and the detailed description of the present invention to follow, is provided for understanding the context and applicability of the present invention, and is provided without being bound by any particular theory or the like. References to particular background patents or other materials are for general discussion purposes only and based on Applicants"" understanding thereof, and the complete references should be consulted for the actual contents of such patents and materials.
The present invention provides new gas-phase methods for reducing NOx emissions and other contaminants in exhaust gases, and for industrial processes using the same. With methods in accordance with the present invention, hydrocarbon(s) autoignite and autothermally heat an exhaust gas so that NH3, HNCO or a combination thereof are effective for selectively reducing NOx autocatalytically. These new autocatalytic methods are distinguished by the self-sustained conversion of reactants when at least one reaction product acts as a catalyst so that the reactions proceed faster with formation of the catalyst and continue until reactants are depleted substantially.
With methods in accordance with preferred embodiments of the present invention, the reduction of NOx is driven by autothermally heating the exhaust gas to generate the effective catalytic species for self-sustaining the reactions until reactants are depleted substantially. Within the temperature range of about 900-1600xc2x0 F., hydrocarbon(s) are introduced to autoignite under generally uniform fuel-lean conditions with about 2-18% O2 in the exhaust gas. Once ignited, the reactions proceed autocatalytically, heating the exhaust gas autothermally. Under some conditions, a blue chemiluminescence may be visible.
Such single-stage, autocatalytic methods in accordance with the present invention need not depend on the order in which the reductant(s) and hydrocarbon(s) are introduced into the exhaust gas. Contrary to previous teachings, autocatalytic methods in accordance with the present invention do not require fuel-rich combustion or multiple reaction stages. The NH3, HNCO or a combination thereof may be introduced or generated from reductant(s) before or during the fuel-lean autothermal conversion of hydrocarbon(s) and CO in the exhaust gas.
Autocatalytic methods in accordance with the present invention reduce NOx and deplete both CO and NH3 in a substantially concurrent manner. These autocatalytic reactions are self-sustained by the autothermal heating of the exhaust gas following the substantially uniform autoignition of the hydrocarbon(s). Gas-phase methods in accordance with the present invention may be advantageously applied without a solid catalytic surface. Self-sustaining autothermal reactions in the gas phase may serve to partially remove other exhaust gas contaminants, including hydrocarbons, particulate matter and CO.
Methods in accordance with the present invention may be considered to combine advantages of known methods for reducing NOx selectively, but, unexpectedly, without the disadvantages of solid catalytic surfaces, hazardous wastes or byproduct emissions, etc. Contrary to previous teachings, autocatalytic methods in accordance with the present invention interchangeably may use reductant(s) that consist of or decompose to generate NH3, HNCO or a combination thereof. In addition, hydrocarbon(s) that may be used in embodiments of the present invention may consist of the same liquid, gaseous or vaporous fuels that are combusted to produce the exhaust gas containing NOx in the industrial process.
Also contrary to previous teachings, hydrocarbons and CO do not serve to lower the effective temperature range for reducing NOx by the autocatalytic method. With autocatalytic methods in accordance with the present invention, the exhaust gas is heated autothermally by both the partial oxidation of hydrocarbon(s) to generate CO and the oxidation of CO to CO2. The introduction of NH3 HNCO or a combination thereof during this autothermal heating results in NOx reduction, and the hydrocarbon(s), CO and NH3 are depleted together in the same temperature range of about 1400-1550xc2x0 F. Within this range, the depletion of hydrocarbon(s), CO and NH3 depends primarily on the final temperature for autothermal heating of the exhaust gas.
Also contrary to previous teachings, the autocatalytic method is not limited by the inhibition of CO oxidation. Autocatalytic reactions may be self-sustained while CO and NH3 are depleted together when hydrocarbon(s) autoignite and heat the exhaust gas autothermally to the temperature range of about 1400-1550xc2x0 F. In accordance with the present invention, NH3 may be depleted below even 2 ppm concurrently with CO removal below about 50 ppm.
Also contrary to previous teachings, NOx emissions are reduced to low levels while the NH3 is depleted substantially. In accordance with autocatalytic methods of the present invention, NOx emissions may be reduced about 80-90% to about 50-200 ppm using NH3 and HNCO nearly stoichiometrically. Furthermore, in preferred embodiments NOx emissions may be reduced by as much as 99% to levels as low as about 10 ppm using no more than about twice the stoichiometric ratio of NH3 and HNCO relative to NOx.
Such uniquely concurrent gas-phase removal of NOx, NH3, HNCO, hydrocarbon(s) and CO in general is not highly dependent on the chemical reductant(s). Similar results have been obtained in accordance with the present invention using NH3, cyanuric acid, urea or decomposition products of urea. While the conversion of NOx to N2O may depend on the chemical reductant(s), if desired byproduct N2O emissions may be reduced to low levels using NH3 rather than other chemical reductant(s).
In preferred embodiments of the present invention, the introduction of hydrocarbon(s) is controlled to maintain a final reaction temperature in the range of about 1400-1550xc2x0 F. The autothermal heat release increases the exhaust gas temperature adiabatically in the absence of heat losses, or alternatively heat transfer surfaces may recover heat from the exhaust gas during the autothermal heating. Such heat recovery, however, should not cool the exhaust gas so excessively as to extinguish the autothermal reactions.
An autothermal heat release equivalent to an adiabatic temperature increase in the range of about 50-500xc2x0 F. is preferentially utilized in preferred embodiments to achieve a final exhaust gas temperature in the range of about 1400-1550xc2x0 F. for implementing autocatalytic methods in accordance with the present invention. The amount of hydrocarbon(s) introduced depends primarily on the initial exhaust gas temperature and any recovery (recycling) of heat released by the autothermal reactions.
Autocatalytic methods in accordance with the present invention typically utilize residence times no longer than about 1.5 seconds when the initial exhaust gas temperatures are in the range of about 900-1600xc2x0 F. As more fully describe elsewhere herein, CO and NH3 typically are depleted faster when the autothermal heating is initiated at higher temperatures in the range of 1050-1600xc2x0 F. In this case, reaction residence times in the range of about 0.02-1.0 seconds typically may be sufficient to deplete both CO and NH3 substantially. In accordance with the present invention, higher initial exhaust gas temperatures in the range of about 1200-1600xc2x0 F. enable substantial CO and NH3 depletion within the range of about 0.02-0.5 seconds.
In accordance with the present invention, the introduction of hydrocarbon(s) decreases beneficially when the exhaust gas is preheated to the temperature ranges of about 1050-1600xc2x0 F. or about 1200-1600xc2x0 F. In these cases, the autothermal heat release need not exceed an amount equivalent to an adiabatic increase of about 50-350xc2x0 F. or about 50-200xc2x0 F., respectively, so long as the exhaust gas is heated autothermally to a final temperature in the range of about 1400-1550xc2x0 F. In accordance with the present invention, in certain embodiments this preheating of the exhaust gas also may improve the selectivity of NOx reduction.
With the present invention, the initial exhaust gas temperatures do not depend on how the exhaust gas is preheated or cooled, so long as the O2 concentration is maintained in the range of about 2-18% by volume. The exhaust gas may be heated or cooled initially using heat transfer surfaces, including any of various methods for preheating the exhaust gas by recovering heat after the exhaust gas is treated using autocatalytic methods as provided herein. In alternative embodiments, the exhaust gas is heated directly by the combustion of a supplemental fuel in the exhaust gas.
In such alternative embodiments, the combustion of a supplemental fuel using excess air also may enrich the O2 concentration in an otherwise 02-deficient exhaust gas. In this case, the supplemental fuel combustion may serve the dual purpose of preheating the exhaust gas and enriching its O2 concentration. The combustion of a supplemental fuel also may serve to preheat a portion of the exhaust gas to ignite more supplemental fuel which is combusted directly in the exhaust gas. If the exhaust gas is preheated using fuel-rich combustion, autocatalytic methods in accordance with the present invention may serve to remove in part or substantial whole the additional contaminants from the fuel-rich combustion. In this context, it is important to note that the xe2x80x9csupplemental fuelxe2x80x9d is combusted for the purpose of preheating the exhaust gas, and not for chemically enhancing the NOx reduction.
Autocatalytic methods in accordance with the present invention may be used in combination with various modifications to the combustion process which generates the exhaust gas. In certain embodiments, such modifications may advantageously lower NOx emissions to decrease the introduction of reductant(s) in accordance with the autocatalytic methods of the present invention. In certain embodiments, combustion modifications may beneficially maintain exhaust gas temperatures within the range of about 900-1600xc2x0 F., or preferably about 1200-1600xc2x0 F. for implementing autocatalytic methods of the present invention. In certain embodiments, combustion modifications may maintain the O2 concentration above about 2% by volume for implementing autocatalytic methods of the present invention.
Autocatalytic methods in accordance with the present invention also may be implemented in conjunction with the primary combustion process so that the autothermal heat release is recovered beneficially. In alternative embodiments, for example, existing or new surfaces in a heat exchange boiler may serve to recover the autothermal heat release generated in accordance with autocatalytic methods as provided herein. In such embodiments, the autothermal heating may replace primary fuel for the purpose of generating steam or cracking petrochemicals (as exemplary industrial applications), or the autothermal heating may serve to increase the generating capacity of an existing boiler, etc.
In accordance with still other embodiments, combustion modifications such as over-fire air are utilized to enable lower NOx emissions from the primary fuel while also enriching O2 in the exhaust gas. In the case of coal-fired boilers, the replacement of primary fuel by autothermal oxidation may serve to increase furnace O2 levels beneficially for the purpose of decreasing unburned carbon on fly ash. Such benefits of alternative embodiments of autocatalytic methods in accordance with the present invention may serve to increase overall boiler efficiency while also enhancing the value of byproduct fly ash, possibly avoiding the generation of an otherwise solid waste.
Autocatalytic methods in accordance with the present invention also may beneficially consume NH3 breakthrough from a previous exhaust gas treatment using SNCR, for example. In such embodiments, autocatalytic methods in accordance with the present invention may serve to replace the use of SCR as a means for controlling NH3 breakthrough from SNCR. Such uses of embodiments of the present invention, however, also may preferably apply autocatalytic methods as provided herein in place of SNCR in order to reduce NOx more selectively. It is submitted that, as one exemplary advantage, the better selectivity of autocatalytic methods as provided herein may greatly decrease the introduction and cost of reductant(s), while substantially depleting both CO and NH3, and reducing NOx emissions to low levels.
Since autocatalytic methods in accordance with the present invention may reduce NOx emissions below most regulatory requirements, application of the present invention may replace the need for expensive catalysts altogether. Such autocatalytic activity for removing NH3 and CO along with NOx may be self-sustained and conducted in a manner so as to not deteriorate with use like solid catalysts. As a result, autocatalytic methods in accordance with the present invention may avoid the need to replace existing catalysts poisoned by exhaust contaminants.
If emissions regulations require additional NOx reductions, then autocatalytic methods in accordance with the present invention may serve to enhance SCR applications while minimizing catalyst volume. In addition to reducing NOx before SCR, autocatalytic methods as provided herein also may decrease contaminants such as hydrocarbons and soot which may foul catalytic surfaces. In such embodiments utilizing a combination of treatments, autocatalytic methods as provided herein may enable the use of more efficient or cost-effective catalyst beds due to both contaminant removal and the control of exhaust gas temperatures.
In a preferred combination of autocatalytic and catalytic reduction methods, autothermal heating may continuously decrease hydrocarbon and soot contaminants while controlling the exhaust gas temperature to the catalyst. Emissions of NOx may be maintained at a desired level using a separate injection of NH3 ahead of the catalyst to minimize reductant(s) introduced to the autocatalytic method. Since autocatalytic NOx reduction need not exceed about 80-90%, reductant(s) may be consumed nearly stoichiometrically, and CO emissions may be substantially depleted in the shortest time possible.
Accordingly, it is an object of the present invention to address problems, limitations and disadvantages of prior techniques of NOx reduction from exhaust gases produced by a variety of industrial processes.
It is another object of the present invention to provide practical and low-cost methods of NOx removal in a variety of industrial processes, which may utilize a variety of commercially-available reductants.
It is another object of the present invention to provide practical and low-cost methods is of NOx removal in a variety of industrial processes, which may deplete NH3 and CO in a substantially concurrent manner.
It is yet another object of the present invention to provide NOx reduction methods which do not require solid catalytic surfaces or hazardous materials.
It is a further object of the present invention to provide methods of NOx removal that may be selective and conducted nearly stoichiometrically in the gas phase.
Finally, it is an object of the present invention to provide autothermal, autocatalytic NOx reduction methods in a wide variety of industrial processes, and also to various systems and apparatus for carrying out the same.